Question

consider ammonia molecules.find all the Raman active modes(in terms of IRs)​

Answer 1

Answer:

If your molecule has no symmetry element, all modes will be Raman active. (The intensity is another question). If you molecule has symmetry, those vibrations will be Raman active which transform similarly to the xy, yz or xz products. You can use the character tables to see how these products transform using various symmetry elements. Most books on vibrational spectorscopy contain these character tables. The vibrational modes for not too large molecules can be calculated by force field analysis (quantum chemistry).